<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .center {
            text-align: center;
        }

        h3.bg-green {
            background-color: green;
        }

        .red {
            color: red;
        }

    </style>
</head>
<body>
<h3>The Class Selector</h3>
<h4 class="center">Hello</h4>
<p class="center">This paragraph is.</p>
<hr>
<h3 class="bg-green">Hello World</h3>
<p class="bg-green">This paragraph is not affected by the style.</p>
<hr>
<h3 class="center red">Hello World</h3>
<p class="red">This paragraph is not affected by the style.</p>
</body>
</html>